FIG. 1 depicts a schematic diagram of a portion of a wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a wireless telecommunications system is a wireless switching center ("WSC"), which also may be known as a mobile switching center or mobile telephone switching office. Typically, a wireless switching center (e.g., WSC 120) is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic region serviced by the system and to the local and long-distance telephone and data networks (e.g., local-office 130, local-office 138 and toll-office 140). A wireless switching center is responsible for, among other things, establishing and maintaining a call between a first wireless terminal and a second wireless terminal or, alternatively, between a wireless terminal and a wireline terminal (e.g., wireline terminal 150), which is connected to the system via the local and/or long-distance networks.
The geographic region serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called "cells." As depicted in FIG. 1, each cell is schematically represented by a hexagon. In practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with the wireless switching center.
For example, when a user of wireless terminal 101-1 desires to transmit information to a user of wireless terminal 101-2, wireless terminal 101-1 transmits a data message bearing the user's information to base station 103-1. The data message is then relayed by base station 103-1 to wireless switching center 120 via wireline 102-1. Because wireless terminal 101-2 is in the cell serviced by base station 103-1, wireless switching center 120 returns the data message back to base station 103-1, which relays it to wireless terminal 101-2.
In a terrestrial wireless telecommunications system, in contrast to a satellite-based system, an empirical phenomenon known as multipath fading affects the ability of a base station and a wireless terminal to communicate. The cause of multipath fading and the factors that affect its severity are described below.
FIG. 2 depicts an illustration that aids in understanding the cause of multipath fading. When a base station transmits a signal to a wireless terminal with either a directional or an omni-directional antenna at least some images of the signal radiate in a direction other than directly at the wireless terminal. The result is that: (1)one image of the signal may be received by the wireless terminal in a direct, line-of-sight path, provided that one exists (e.g., image 202-3), (2)other images of the signal pass the wireless terminal and are never received (e.g., images 202-2 and 202-4), and (3)other images of the signal strike an object, such as a building, and are reflected or refracted towards the wireless terminal (e.g., images 202-1 and 202-5). The result is that an image of a transmitted signal can be received by a wireless terminal via a direct path and one or more indirect paths.
Furthermore, the signal quality (as measured by, for example, the signal-to-noise ratio, average power, absolute power, frame-error rate, bit-error rate, etc.) of each image varies as a function of the length of the path, whether the signal is reflected off or refracted through an object, the angle at which the signal is incident to the object, and the geometric and physical properties of the object.
Because each image travels at the same speed (i.e., the speed of light) over a different length path, each image arrives at the wireless terminal at a different time. This causes the various images to arrive out of phase with respect to each other, and thus, to interfere. When the interference is destructive, in contrast to constructive, the interference greatly hinders the ability of a wireless terminal to generate an acceptable estimate of the transmitted signal. The phenomenon of destructive interference by multiple phase-shifted images of a single transmitted signal is known as multipath fading.
The severity of multipath fading at a receive antenna is a function of three factors: (1)the location of the transmitting antenna with respect to the objects in the environment that reflect and refract the transmitted signal, (2)the location of the receive antenna with respect to the same objects, and (3)the wavelength of the transmitted signal. Because these factors are spatial in nature, multipath fading is a localized phenomenon. In other words, multipath fading occurs in isolated pockets called "fades" that are geographically dispersed. As an analogy, fades are isolated and dispersed throughout a geographic region like the holes are isolated and dispersed in Swiss cheese. Typically, the mean diameter of a fade equals one wavelength of the transmitted signal.
There are two techniques in the prior art for mitigating the effect of multipath fading and both are derived from an understanding that the phenomenon is localized in nature. The first technique, receive diversity, will be discussed first, and then the second technique, transmit diversity, will be discussed.
In accordance with receive diversity, a radio receiver employs two receive antennas that are positioned far from each other to receive a signal that is transmitted from only one antenna. Typically, the two receive antennas are positioned more than several wavelengths of the transmitted signal from each other. Because multipath fades are isolated and dispersed like the holes in Swiss cheese, roughly circular in shape and about one wavelength of the transmitted signal in diameter, it is unlikely that both receive antennas will be in a fade at the same time. In other words, if one antenna is in a fade, then it is unlikely that the other is also in a fade. Therefore, the radio receiver can operate with the confidence that the transmitted signal will be received with satisfactory quality at one of the receive antennas.
FIG. 3 depicts a block diagram that illustrates how receive diversity can be implemented in the wireless telecommunications system of FIG. 1. In FIG. 3, base station 103-1 transmits a signal via one transmit antenna, Tx, to wireless terminal 101-1, which has two receive antennas Rx.sub.1 and Rx.sub.2 that are separated by several wavelengths of the transmitted signal. Although the arrangement in FIG. 3 mitigates the effect of multipath fading, it is generally impractical to mount two antennas on a wireless terminal when the antennas need to be more than a few inches apart. Furthermore, the need for two antennas on a wireless terminal greatly increases its cost. It is for these reasons that receive diversity is rarely implemented in wireless terminals.
Transmit diversity is a corollary of receive diversity. In accordance with transmit diversity, a radio transmitter employs two transmit antennas that are positioned far from each other to transmit one signal. The radio receiver only has one antenna. Typically, the two transmit antennas are positioned more than several wavelengths of the transmitted signal from each other. The radio transmitter outputs the signal of interest via one antenna in real-time, and delays an exact copy of the same signal before outputting it via the second antenna. Because the location of a multipath fade is dependent on the location of the transmitting antenna, each transmit antenna causes fades to occur in different places. Therefore, if a receive antenna is in a fade caused by the signal from one transmit antenna, it is likely that the receive antenna will be able to receive the signal from the other transmit antenna with satisfactory quality. In other words, it is unlikely that both transmit antennas will cause fades in the same place, and therefore, the radio receiver is likely to be able to receive the signal from at least one of the transmit antennas at any given location.
FIG. 4 depicts a block diagram that illustrates how transmit diversity can be implemented in the wireless telecommunications system of FIG. 1. In FIG. 4, base station 103-1 transmits a signal via one transmit antenna, T.sub.x1, in real-time, and delays an exact copy of the same signal before outputting it via a second antenna, T.sub.x2. Transmit diversity in the prior art is disadvantageous, however, because it creates, on average, twice as many images of the transmitted signal at the receiver than without transmit diversity. This requires that the wireless terminal be capable of distinguishing the two time shifted images, which significantly increases the complexity of the wireless terminal and also its cost.
Therefore, the need exists for a technique for mitigating the effect of multipath fading without some of the costs and disadvantages associated with the prior art.